Figure 9. An alternative radiosurgical plan based on volumetric modulated arcs using a 5-mm leaf-width multileaf collimator.

The images show (A, B) the dose distributions superimposed onto an axial CT image and (C) the DVHs.

(A, B) The representative isodoses are shown similarly to Figures 2A, 2B (the original plan). (A-C) The alternative plan for the initial SRS was generated using volumetric modulated arcs with the Agility^® collimator (Elekta AB, Stockholm, Sweden), where 52.4 Gy was assigned to the D_99% of the GTV as per the original plan. No constraint to the internal maximum dose of the GTV results in more concentrically laminated, steeper dose increase inside the GTV boundary (more inhomogeneous GTV dose) and more precipitous dose falloff (gradient) outside the GTV: the D_99% of the GTV + 3 mm (36.0 Gy) is reduced by 17% compared to that of the original plan (43.3 Gy). The cumulative BED₂s of the GTV + 3 mm D_99% in the original and alternative plans are 157.1 Gy and 130.5 Gy (17% reduction), respectively. Thus, the alternative plan can attenuate the risk of relevant adverse radiation effects and provide room for GTV dose escalation without increasing the normal tissue dose outside the GTV.

D_99%: a minimum dose encompassing at least 99% of the target volume; GTV: gross tumor volume; GTV + 3 mm: GTV plus a uniform 3-mm margin; CT: computed tomography; DVH: dose-volume histogram; BED_10/2: a biologically effective dose based on the linear-quadratic formula with an alpha/beta ratio of 10/2